1. Field of the Invention
This invention relates to a reflective optical element and a reflective optical system that are suitable for optical apparatus such as video cameras, still cameras, head-mounted displays, and finders.
2. Description of Related Art
An example of the reflective optical system is proposed in Japanese Laid-Open No. H08-292371. FIG. 13 shows the optical system proposed in this publication.
In FIG. 13, a light flux emitted from an object passes through an aperture-stop S and enters a reflective optical element B101. The light flux incident on the reflective optical element B101 is refracted by a first surface R101, is then reflected by a second surface R102, a third surface R103, a fourth surface R104, a fifth surface R105 and a sixth surface R106, is then refracted by a seventh surface R107, and is emitted from the reflective optical element B101.
In the reflective optical element B101, the light flux emitted from the object forms a primary image on an intermediate image-formation surface near the second surface R102, and forms an image of the pupil near the fifth surface R105. The light flux emitted from the reflective optical element B101 finally forms an image on an image pickup surface (e.g., image pickup surface of an image pickup device like a CCD or CMOS) IS.
This reflective optical system uses the reflective optical element B101 in which a plurality of curved and flat reflecting surfaces are integrally formed, thereby achieving a size reduction of the whole reflective optical system, and excluding the influence of mirror arrangement accuracy (assembly accuracy) upon an optical performance which is liable to cause problems in the reflective optical system using reflecting mirrors.
Additionally, the aperture-stop S is disposed on the side nearest to the object in the optical system, and an object image is formed at least once in the reflective optical element B101, thereby enabling a reduction in the effective diameter of the optical element although it is a reflective optical element with a wide field angle.
Further, appropriate refractive power is given to the plurality of reflecting surfaces that constitute the reflective optical element, and each reflecting surface is arranged in a decentering manner, thereby bending the optical path in the optical system into a desired shape, and achieving a shortening of the whole length in a predetermined direction of the optical system.
This decentered optical system is called an off-axial optical system. In greater detail, on the assumption that an axis along a light beam passing through a center of an image and through a center of a pupil is a reference axis, this optical system is defined as an optical system that includes a curved surface (off-axial curved surface) whose normal at an intersection with a reference axis does not exist on the reference axis and in which the reference axis forms a bent shape.
In this off-axial optical system, the constructive surface generally is decentered surface, and an eclipse never occurs in the reflecting surface. Therefore, it is easy to construct an optical system using reflecting surfaces. Japanese Laid-Open No. H8-292372, Japanese Laid-Open No. H9-222561, and Japanese Laid-Open No. H9-258105, etc. propose a variable-power optical system that uses these optical elements, and Japanese Laid-Open No. H9-5650, etc. propose its design method.
In a reflective optical system proposed in Japanese Laid-Open No. H8-292371, an intermediate image is formed inside, in order to make the effective diameter of an optical surface in an optical element smaller. Therefore, the optical path length tends to inevitably lengthen, and the optical element tends to extend longitudinally.
Since this optical element has a great degree of freedom to arrange reflecting surfaces, a length in a certain direction can be reduced, nevertheless the whole volume increases.
If an optical path merely intersects as in an optical element proposed in Japanese Laid-Open No. H11-064734, each reflecting surface is great in size, and each effective diameter has almost the same length. Further, in order to make this optical element compact, there is a need to design normals of three or more reflecting surfaces to face each other, and there is a need to cause the optical path to intersect two or more times in an area enclosed by the plurality of reflecting surfaces.
FIG. 14 is a typical drawing of an optical element in which three reflecting surfaces, each of which has almost the same effective diameter, are arranged so that normals of these surfaces face each other. A light flux emitted from an object is reflected by the reflecting surfaces R201, R202, R203, and thereafter must pass between the reflecting surface R201 and the reflecting surface R202.
However, since the distance between the reflecting surfaces R201 and R202 is narrow, the light flux cannot pass therebetween. The distance between the reflecting surfaces R201 and R202 must be increased, or, alternatively, the size of the reflecting surface R202 must be reduced, in order to cause the light flux to pass therebetween.
Since an increase in the distance between the reflecting surfaces R201 and R202 is contrary to a size reduction of the optical element, it is necessary to reduce the size of the reflecting surface R202.
FIG. 15 shows an example in which the size of the reflecting surface R202 is reduced more than in FIG. 14. In this example, the distance between the reflecting surfaces R201 and R202 is sufficiently great, and therefore the light flux reflected by the reflecting surface R203 can be caused to pass between the reflecting surfaces R201 and R202.
The reflective optical element constructed in this way can be used for an observation optical system of, for example, a head-mounted display or an optical finder of a camera. Also in this case, large magnification and size reduction of the reflective optical element are strongly demanded for the performance improvement and size reduction of the head-mounted display and camera.
It is an object of the present invention to provide a reflective optical element in which an optical path can intersect two or more times by use of three or more reflecting surfaces in spite of being compact and in which a light flux can be easily emitted from an area enclosed by these reflecting surfaces, and provide a reflective optical system and an optical apparatus that use the reflective optical element.
In order to achieve the object, the reflective optical element of the present invention is structured as follows. That is, the reflective optical element has three or more reflecting surfaces by which a light flux from an object is successively reflected, and a reference axis intersects at least two times and the light flux forms an intermediate image in an area enclosed by the reflecting surfaces. The reference axis is a path of the light beam which passes through the center of an object surface, is then reflected by the reflecting surfaces, and passes through a center of a pupil. The reflective optical element satisfies the following condition:
4xc2x7fxc2x7tanxcex8 less than eaxe2x80x83xe2x80x83(1) 
wherein xcex8 is a maximum field angle that passes through the pupil in a plane that includes the reference axis, f is a focal length of an optical part between the pupil and the intermediate-image, and ea is a maximum one of optical effective diameters of surfaces which the reflective optical element has.
If the upper limit of the condition (1) is exceeded, the minimum optical effective diameter in the plane that includes the reference axis becomes greater than half the maximum optical effective diameter, and, as a result, each reflecting surface becomes greater, and therefore it becomes difficult to emit a light flux from between the reflecting surfaces in the area enclosed by the reflecting surfaces.
In order to achieve the object, the finder optical system of the present invention for guiding a light flux from an object to the eye of an observer is structured as follows. That is, the finder optical system has a reflective optical element that includes a first reflecting surface by which a light flux from the object is reflected, a second reflecting surface by which the light flux reflected by the first reflecting surface is reflected, and a third reflecting surface by which the light flux reflected by the second reflecting surface is reflected. In an area enclosed by the first to third reflecting surfaces, a reference axis has two crossed points at a position where the optical path from the second reflecting surface to the third reflecting surface intersects and at a position where the optical path from the third reflecting surface to the pupil intersects with the optical path from the object surface to the first reflecting surface, and the light flux forms an intermediate image in an area enclosed by the first to third reflecting surfaces. The reference axis is a path of a light beam, which is emitted from the center of an object surface, is then reflected by the first, second, and third reflecting surfaces, and passes through the center of a pupil.
A detailed configuration of the reflective optical element, reflective optical system, finder optical system, and optical apparatus of the invention, the above and other objects and features of the invention will be apparent from the embodiments, described below.